The present invention relates to a method for forming images useful for a negative working image-forming material which can directly be formed into a printing plate by scanning a visible laser beam based on digital signals outputted from, for instance, a computer or which can be subjected to the so-called direct plate-making operation.
As a material for preparing lithographic printing plates, there has conventionally and widely been used a PS plate having such a structure that a lipophilic light-sensitive resin layer is formed on a hydrophilic substrate and a desired printing plate has currently been prepared from such a PS plate by a plate-making method which comprises the steps of subjecting the PS plate to a mask-exposure through a lith film and then removing the non-image area of the light sensitive layer on the exposed PS plate through dissolution.
Recently, there have widely been used digitization techniques in which image information is electronically treated, accumulated and outputted by the action of a computer and a variety of corresponding new image-outputting systems have been put to practical use. Consequently, it has been an important technical subject to develop a CTP technique for directly preparing a printing plate by scanning light rays having highly directional properties such as a laser beam in accordance with the digitized image information without using any lith film and to likewise develop an original plate for preparing a printing plate (printing plate precursor or PS plate for preparing a printing plate) suitably used for the CTP technique.
As such light sources, there have been adopted an Ar laser (488 nm), an FD-YAG laser (532 nm) and an InGaN type semiconductor laser (violet laser; 350 to 450 nm) and there has already been developed and put to practical use an original plate for preparing a lithographic printing plate, which comprises a highly sensitive photopolymerizable light-sensitive layer quite susceptible to light rays from such a light source and capable of ensuring a considerable productivity.
Most of the sensitivity-improving techniques relate to photopolymerization initiator systems and examples of such initiator systems highly sensitive to, for instance, Ar lasers and FD-YAG lasers include combinations of titanocene with coumarin dyes (Patent Article 1: Japanese Un-Examined Patent Publication Hei 9-268185; Patent Article 2: Japanese Un-Examined Patent Publication Hei 6-192309; Patent Article 3: Japanese Un-Examined Patent Publication Hei 10-204085), combinations of styryl dyes with titanocene (Patent Article 4: Japanese Un-Examined Patent Publication Hei 9-80750) and combinations of dyes having acidic 5-membered heterocyclic nuclei with titanocene (Patent Article 5: Japanese Un-Examined Patent Publication Hei 10-101719).
In addition, as photopolymerization initiator systems highly sensitive to light rays emitted from violet lasers, there have been known, for instance, combinations of naphtho-furanone type dyes with titanocene (Patent Article 6: Japanese Un-Examined Patent Publication Hei 2000-206690) and combinations of carbazolyl-styryl type dyes with titanocene (Patent Article 7: Japanese Un-Examined Patent Publication Hei 2001-42524).
On the other hand, the PS plate should have a high solubility difference (discrimination) between the exposed and un-exposed areas on the light-sensitive layer thereof as a developing characteristic property in order to provide a lithographic printing plate stably showing the desired functions thereof. It has been known that this problem may be solved to some extent by the addition of a specific surfactant to a developer (Patent Article 8: Japanese Un-Examined Patent Publication Hei 2003-43703), but it would further be important to reduce any damage of image areas and to steadily maintain the stable activity of a developer used, for the further improvement of the quality of the resulting printing plate.
In this respect, there has been reported the use of a strongly alkaline aqueous solution having a pH value of higher than 12.5 and comprising, for instance, potassium silicate for the development of a negative-working image-forming material which makes use of a photopolymerizable composition (Patent Article 9: Japanese Un-Examined Patent Publication Hei 8-108621), but the use of a developer having such a high pH value suffers from problems such that the developer may considerably damage image areas and the resulting printing plate has insufficient printing durability. Moreover, if such a developing treatment is continued over a long period of time, the developer may undergo a change in its ability of absorbing carbon dioxide due to environmental changes such as a change in the environmental carbon dioxide concentration and the developer correspondingly causes a change in the activity thereof. As a result, various problems arise such that the quality of the plate-making material is quite susceptible to such changes and that components of the plate-making material are accumulated in a developing tank and that the accumulation thereof leads to, for instance, the clogging of piping.
Over against it, there has been proposed a method for developing a layer of a light-sensitive resin composition formed on the surface of a glass surface with, for instance, a mixed aqueous solution containing sodium carbonate and sodium hydrogen carbonate, having a pH value of about 10 in order to inhibit any reduction of the developing ability of the developer with the elapse of time and due to the repeated use of the same (Patent Article 10: Japanese Un-Examined Patent Publication Hei 5-88377; Patent Article 11: Japanese Un-Examined Patent Publication Hei 11-65126).
When the inventors of this invention applied these developers to an original plate (PS plate) for preparing a lithographic printing plate which comprises an aluminum substrate provided thereon with a light-sensitive layer containing a photopolymerizable light-sensitive resin composition, however, it was found that the developers did not have any satisfactory ability to develop non-image areas of such a light-sensitive layer. Moreover, there has also been reported an alkaline developer to which an alkyl phenolic surfactant is added for the solution of a problem concerning the ability of developing a photoresist comprising a pigment dispersed therein (Patent Article 12: Japanese Un-Examined Patent Publication Hei 10-239858), but there has not yet been developed any developer which can satisfy the both requirements for the developing ability and the ability to produce a printing plate having sufficient durability.